The present invention relates to a fuel injection method for an internal combustion engine, injection being implemented either in homogeneous normal operation or in inhomogeneous stratified operation.
Although it is applicable to any fuels and engines of any vehicles, the present invention and its underlying problems are explained in relation to gasoline direct injection of an internal combustion engine of a passenger car.
In such internal combustion engines with gasoline direct injection, charge stratification is implemented in the combustion chamber, which will be identified below as stratified operation, for the complete utilization of the consumption advantage in certain operating ranges.
In normal operation of gasoline direct injection, the entire combustion chamber is homogeneously filled with a stoichiometric air-fuel mixture which is ignited by the ignition spark at the moment of ignition.
In stratified operation, however, only a small stoichiometric cloud is introduced into the combustion chamber, the cloud being ignitable locally while the remaining contents of the combustion chamber cannot be ignited. The advantage of such stratified operation lies in the expanded lean operation of the internal combustion engine and consequently an ultimate saving of fuel.
Accordingly, it is desirable to make the operating range of the stratified operation as great as possible, extending it in particular to as high loads and speeds as possible.
The problem underlying the present invention is generally that a fixed moment of ignition is necessary to be able to ignite the stratified charge in the combustion chamber reliably. At certain operating points, this fixed moment of ignition may possibly be very early, typically up to a crankshaft position of 50xc2x0 in advance of top dead center (TDC).
In this connection, it has proven to be disadvantageous that combustion knock may occur with such operating points in stratified unthrottled operation. Since the ignition angle cannot be varied, i.e., adjusted to a xe2x80x9claterxe2x80x9d value or one that is closer to TDC, other measures are required to eliminate any combustion knock that may occur.
In contrast to the known approaches, the fuel injection method according to the present invention has the advantage that the internal combustion engine is operated as far as possible in the fuel-saving stratified operation and is only switched over to homogeneous normal operation if combustion knock (for example, at high intake air and/or coolant temperatures) makes this absolutely necessary since otherwise there would be a danger of damaging parts of the engine.
The idea underlying the present invention is that a switchover from inhomogeneous stratified operation to homogeneous normal operation is made when combustion knock occurs. If a knock should still be present after the switchover and throttling in homogeneous normal operation, an additional possible reaction is to adjust the ignition angle, which is not possible in inhomogeneous stratified operation
According to an example embodiment, the knock intensity is detected as a characteristic value for knock in at least one cylinder of the internal combustion engine in inhomogeneous stratified operation and at least the one cylinder is switched over from inhomogeneous stratified operation to homogeneous normal operation if the knock intensity of the one cylinder exceeds a predetermined first knock intensity threshold.
According to another example improvement, the knock frequency is detected as a characteristic value for knock in multiple cylinders of the internal combustion engine in inhomogeneous stratified operation and multiple cylinders are switched over from inhomogeneous stratified operation to homogeneous normal operation if the knock frequency of the multiple cylinders exceeds a predetermined first knock frequency threshold.
According to another example development, a throttling in homogeneous normal operation is implemented after the switchover, the characteristic value for knock is detected in at least one cylinder of the internal combustion engine in homogeneous normal operation and the ignition angle is adjusted if the detected characteristic value of the one cylinder meets a predetermined second criterion, preferably with regard to knock intensity or knock frequency.
According to another example development, the homogeneous normal operation is implemented during an operating point-dependent time interval and at least the one cylinder is switched over from homogeneous normal operation to inhomogeneous stratified operation.
According to another example development, a characteristic value for knock is detected in at least one cylinder of the internal combustion engine in homogeneous normal operation and at least the one cylinder is switched over from normal homogeneous operation to inhomogeneous stratified operation after implementation of homogeneous normal operation during an operating point-dependent time interval only if the detected characteristic value of the one cylinder meets a predetermined third criterion, preferably with regard to knock intensity or knock frequency.
According to another example development, a characteristic curve is determined at which a switchover between homogeneous normal operation and inhomogeneous stratified operation is implemented as a function of at least one engine-specific parameter, preferably speed and load, and at least one part of the characteristic curve is shifted in response to the fact that at one operating point, for which the inhomogeneous stratified operation is provided, it is determined that the detected characteristic value of the one cylinder meets a predetermined fourth criterion, preferably with regard to knock intensity or knock frequency.
According to another example development, one interpolation point of the characteristic curve corresponding to the operating point is shifted by a predetermined first amount to an adapted interpolation point.
According to another example development, the knock intensity is detected as a characteristic value for knock in the at least one cylinder of the internal combustion engine in inhomogeneous stratified operation and the part of the characteristic curve is shifted if the knock intensity of the one cylinder exceeds a predetermined second knock intensity threshold which is lower than the first knock intensity threshold.
According to another example development, the knock frequency is detected as a characteristic value for knock in multiple cylinders of the internal combustion engine in inhomogeneous stratified operation and the part of the characteristic curve is shifted if the knock frequency of the one cylinder exceeds a predetermined second knock frequency threshold which is lower than the first knock frequency threshold.
According to another example development, the adapted interpolation point of the characteristic curve corresponding to the operating point is shifted back by a predetermined second amount in the direction of the original interpolation point if the detected characteristic value of the one cylinder meets a predetermined fifth criterion, preferably with regard to knock intensity or knock frequency, for a specific time interval, the predetermined second amount preferably being lower than the predetermined first amount.
According to another example development, the cylinders are controllable by individual single throttle valves, and the detection and the switchover is implemented separately for each cylinder.
According to an additional preferred further development, at least one cylinder is throttled in inhomogeneous stratified operation if the detected characteristic value of the one cylinder meets a predetermined sixth criterion, preferably with regard to knock intensity or knock frequency.
According to another example development, the knock intensity is detected as a characteristic value for knock in the at least one cylinder of the internal combustion engine in inhomogeneous stratified operation and at least the one cylinder is throttled in inhomogeneous stratified operation if the knock intensity of the one cylinder exceeds a predetermined third knock intensity threshold which is lower than the predetermined first and second knock intensity threshold.
According to another example embodiment development, the knock frequency is detected as a characteristic value for knock in multiple cylinders of the internal combustion engine in inhomogeneous stratified operation and the multiple cylinders are throttled in inhomogeneous stratified operation if the knock frequency of the multiple cylinders exceeds a predetermined third knock frequency threshold which is lower than the predetermined first and second knock frequency threshold.